An increasingly popular technique for the growth of compound semiconductor epilayers is molecular beam epitaxy (MBE). MBE is extremely versatile, offers atomic monolayer thickness control in the growth direction and the state of art is rapidly being advanced to the point where a commercial rather than a pure research application is feasible. The most immediate commercial application is the growth of AlGaAs/GaAs heterojunction structures from which optoelectronic and microwave devices or very high speed digital integrated circuits can be fabricated. Two serious drawbacks associated with the commercial application of MBE derive from the fact that substrate wafers are typically solder mounted on a refractory metal block during the growth process. First, it is difficult to ensure uniform wetting of the substrate to the solder for large (up to 3" diameter) substrates. Second, the solder reacts with the substrate at the elevated temperatures required for crystal growth, generating substantial damage in the back surface of the substrate. This damage complicates the fabrication of integrated circuits which requires smooth and parallel front and back surfaces and reduces device yield by promoting the propagation of dislocations from the substrate into the epilayer during growth.
In the case of epilayer growth on GaAs substrates by MBE, commercially available substrates are typically 15 to 30 mils thick and at least two inches in diameter. Before being loaded into an MBE system they are mounted onto a molybdenum block coated with a thin film of indium which acts as a solder.
Historically, indium has been the solder of choice because it is a benign isoelectronic impurity in GaAs, it wets readily to GaAs (an alternative, pure gallium, does not) and it has a very low melting point. It has long been observed that indium reacts strongly with the back surface of the substrate creating a damaged nonplanar surface. At temperatures well below typical growth temperatures of GaAs, e.g., 580.degree. C., the substrate dissolves into the indium solder and eventually recrystallizes as an alloy In.sub.x Ga.sub.1-x As with x.ltoreq.0.2 or x.gtoreq.0.8, J. Ding, J. Washburn, T. Sands and V. G. Keramidas, Appl. Phys. Lett., 49 (1986) 818. This implies that the back surface of the substrate after growth will not only be nonplanar but highly strained as well. If the substrate is strained during growth, then removal of the damaged layer on the back surface of the substrate prior to device fabrication will result in a strained and probably dislocated epilayer from which the devices will be fabricated, S. Ysuami, H. ikami and A. Hojo, Jpn. J. Appl. Phys., 22 (1983) 1567.
The kind of damage resulting from the solder alloying with the substrate is visible if the after growth surface of the substrate is etched to selectively remove only the metallic indium adhering to the substrate, revealing a significantly nonplanar surface. Before any device fabrication, integrated circuit or discrete device, the surface roughness must be removed by lapping with an abrasive grit. Extreme care must be taken not to damage the thin epilayer on the front which layer may be as thin as 150 nm. Care must also be taken to achieve parallel front and back surfaces. If the front surface is damaged, device yield suffers. If the two faces are not parallel, then the entire substrate is likely to be broken during the lithographic fabrication process which involves several contact printing steps. It is therefore desirable to maintain a smooth back surface during growth and avoid the necessity of lapping the substrate at all.
In addition, where the substrate is not solder-mounted but held, e.g., by clips, problems in epilayer growth occur if the heating, e.g., by direct radiant heating or infrared illumination, is not uniformly absorbed by the substrate. Furthermore, in the growth of refractory semiconductors on less refractory substrates, e.g., AlAs (grown at temperatures in excess of 800.degree. C.) on GaAs (which decomposes in vacuum at 580.degree. C.), in the absence of an In solder layer, direct radiant heating of the substrate will drive the decomposition of the back surface of the substrate by the rapid preferential evaporation of As from the crystal. In the case of a typical III-V compound semiconductor substrate, the Group V element always shows this type of behavior. It has been proposed in such a situation that the back surface of the substrate be coated with a layer of 2000 .ANG. of sputter-deposited molybdenum. Erickson et al., J. Vac. Sci. Technol. B(3)2, March/April 1985, page 536. This presents additional difficulties, however, since the molybdenum is rapidly removed by the standard H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 /H.sub. 2 O etch used in substrate preparation, thereby necessitating the less desirable sequence of etching the substrate before depositing the molybdenum.